a food processor or mandoline with a shredder attachment
Whisk the Dijon mustard, balsamic vinegar, honey, 1/2 teaspoon salt and a few grinds of black pepper in a medium bowl. Whisking constantly, slowly drizzle in the olive oil until emulsified. Set aside.
Toast the hazelnuts in a medium skillet over medium-high heat, tossing a few times, until golden brown spots start to form, 3 to 4 minutes. Immediately transfer to a cutting board to stop the cooking and roughly chop them.
Use the shredder attachment on a food processor or a mandoline to shave the Brussels sprouts. Put the sprouts in a large bowl and add the hazelnuts, cranberries, 2 teaspoons salt and a bunch of grinds of black pepper. Drizzle the dressing over top (it will feel like a lot of dressing, but trust me, it needs it!) and toss until well combined.